The aim of the proposed research is to determine whether the central nervous system utilizes specific information provided to it by the various types of muscle spindle afferent endings. The results of these studies will provide data relevant to understanding the role of muscle receptors in the maintenance of normal muscle tone, movement disorders and spasticity. The trigeminal system is particularly attractive for these studies because of the complex morphology and mode of afferent terminations that exist in jaw-muscle spindles and their correspondingly diverse afferent responses. If the central nervous system makes specific use of such information, then it is expected that it will be reflected in differences in the central termination pattern and synaptic distribution of these afferents. To test this hypothesis individual rat jaw-muscle spindle afferent axons will be impaled with microelectrodes and their response recorded during controlled ramp and h old muscle stretches, prior to and following the infusion of the depolarizing drug suxamethonium, and when stretched to a fixed length. Horseradish peroxidase (HRP) will then be inonophoresed int the axon and the animal perfused. The brain will then be removed and histochemically processed to allow the visualization and reconstruction of HRP-stained axons. Spindle afferent responses will be classified as primary, secondary or intermediate based upon: a) their dynamic responsiveness during the ramp stretches, b) any increase in dynamic sensitivity following administration of the depolarizing drug suxamethonium and c) their coefficient of variation when stretched to a fixed length. The anatomical projections of physiologically classified spindle afferents will be examined to determine whether: 1) characteristic central distribution patterns exist for each afferent type, 2) all types project monosynaptically to the trigeminal motor nucleus and 3) any types project to common brainstem regions and possibly converge onto shared interneurons.